This present invention relates generally to an apparatus and method for scanning and inspecting baggage. More particularly, the present invention relates to an explosive detection system (EDS).
Carry-on and checked baggage inspection systems generally utilize a scan projection (SP) image for presentation to the operator. In most baggage inspection systems, scan projection images are created by moving an object under a fan beam of x-rays from a stationary x-ray source. X-ray intensities, after being attenuated by the object being scanned, are measured by an array of detectors. The x-ray intensity data is converted through a process called normalization so that each pixel represents approximately the total mass traversed by the ray. SP images may be difficult to interpret because they are an orthographic projection in one direction (the direction of bag travel), but are a perspective projection in the other direction (across the x-ray fan).
In some computed tomography (CT) imaging system configurations, an x-ray source projects a fan-shaped beam which is collimated to lie within an X-Y plane of a Cartesian coordinate system and generally referred to as an “imaging plane”. The x-ray beam passes through an object being imaged. The beam, after being attenuated by the object, impinges upon an array of radiation detectors. The intensity of the attenuated radiation beam received at the detector array is dependent upon the attenuation of the x-ray beam by the object. Each detector element of the array produces a separate electrical signal that is a measurement of the beam intensity at each detector location. The intensity measurements from all the detectors are acquired separately to produce a transmission profile.
In third generation CT systems, the x-ray source and the detector array are rotated with a gantry within the imaging plane and around the object to be imaged such that the angle at which the x-ray fan beam intersects the object constantly changes. A group of x-ray attenuation measurements (e.g., projection data), from the detector array at one gantry angle is referred to as a “view”. A “scan” of the object comprises a set of views made at different gantry angles, or view angles, during one revolution of the x-ray source and detector about the object or patient being imaged.
Many modern CT systems are helical scanners (also known as spiral scanners), in which the scanned object is continually moved while the projection data is being acquired. The path of the X-Ray source describes a helix with respect to the scanned object. Most helical scanners have multiple rows of detectors, and the x-ray fan is collimated into a cone to illuminate the entire array of detectors. The angle between the x-ray source and the first and last detector rows is referred to as the “cone angle”.
The entire scanned volume scanned by the helical scanner can be reconstructed using well known tomographic reconstruction algorithms such as direct Fourier or filtered back projection methods, and more exact methods described by Feldkamp and Katsevich. All of these techniques require a very large amount of computation.
Orthographic and SP-like images can be created from the reconstructed volumetric data by projecting digitally through the reconstructed data. This requires significant additional computation, and the resulting projection may not have as much resolution as the original scan data.
Projection images (also known as radiographic images) are required for EDS operator resolution, and also may be used to select a limited number of planes that need to be reconstructed from the helical data. A helical scanner produces data that can be used to reconstruct a volume, but does not directly produce a readable projection image. Reconstruction of the volume, and then creating a projection through the volume as discussed above requires a very large amount of computation, and the result may have limited resolution.
Accordingly, it is desirable to provide an apparatus and method for creating a projection image directly from the helical scan data.